* `data` can be anything the `fn` callback would want to use.
+* `show_all_errors` tells whether to stop at the first error or not.
+
Initializing
------------
`fill_tree_descriptor`::
- Initialize a `tree_desc` and decode its first entry given the sha1 of
- a tree. Returns the `buffer` member if the sha1 is a valid tree
- identifier and NULL otherwise.
+ Initialize a `tree_desc` and decode its first entry given the
+ object ID of a tree. Returns the `buffer` member if the latter
+ is a valid tree identifier and NULL otherwise.
`setup_traverse_info`::